In conventional encoding and decoding methods for compressing and decompressing video data, a Discrete Cosine Transform (DCT) is applied to encoding of an original picture or a differential picture effectively after performing intra prediction or motion compensated prediction (MCP) onto blocks of a predetermined size. Here, as the block size becomes small, the efficiency of MCP is improved but utility of spatial correlation is decreased. As the block size becomes large, the utility of the spatial correlation can be improved, but prediction errors are relatively increased in the MCP.